For centuries people have speculated about the possibility of life on Mars due to the planet's proximity and similarity to Earth. Although there has been much speculation, to date there has never been any proof of life existing on Mars. However, cumulative evidence is now building that Mars once was habitable.

Serious searches for evidence of life began in the 19th century, and they continue today via telescopic investigations and landed missions. While early work focused on phenomenology and bordered on fantasy, modern scientific inquiry has emphasized the search for water, chemical biosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.[1]

Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks plate tectonics or continental drift, so it has remained almost unchanged since the end of the Hesperian period. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to abiogenesis, even if life does not or has never existed there.[2][3] It remains an open question whether life currently exists on Mars or has existed there in the past, and fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries.

On January 24, 2014, NASA reported that Mars is now a primary NASA objective.[4]

Contents

Early speculation1

Habitability2

Past2.1

Present2.2

Subsurface2.2.1

Surface brines2.2.2

Cosmic radiation2.3

Nitrogen fixation2.4

Low pressure2.5

Liquid water3

Silica3.1

Possible biosignatures4

Methane4.1

Formaldehyde4.2

Viking lander biological experiments4.3

Meteorites4.4

ALH840014.4.1

Nakhla4.4.2

Shergotty4.4.3

Yamato 0005934.4.4

Geysers on Mars5

Forward contamination6

Life under simulated Martian conditions7

Missions8

Mars-28.1

Mariner 48.2

Viking orbiters8.3

Viking experiments8.4

Phoenix lander, 20088.5

Mars Science Laboratory8.6

Future astrobiology missions8.7

See also9

References10

External links11

Early speculation

Mars' polar ice caps were discovered in the mid-17th century. In the latter part of the 18th century, William Herschel proved they grow and shrink alternately, in the summer and winter of each hemisphere. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does — but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.

In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906,[8] proposing that the canals were the work of a long-gone civilization.[9] This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.

Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere.[10] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.

Habitability

Chemical, physical, geological, and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.[11] The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from Solar ultraviolet and galactic cosmic radiation.[12][13]

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below.[11] Similarly, for each group of parameters, the habitability threshold for each is to be determined.[11] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.[14] There are no full-Mars simulations published yet that include all of the biocidal factors combined.[14]

Past

Recent models have shown that, even with a dense CO2 atmosphere, early Mars was, in fact, colder than Earth has ever been.[15] However, transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid–late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars.[15] Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid Hesperian onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both.[15]

The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.[17] When there was a magnetic field, the atmosphere would have been protected from erosion by solar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.[18] The loss of the atmosphere was accompanied by decreasing temperatures. A part of the liquid water inventory sublimed and was transported to the poles, while the rest became trapped in a subsurface ice layer.[15]

Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long lasting hydrothermal system when ice is present in the crust. For example, a 130 km large crater could sustain an active hydrothermal system for up to 2 million years, that is, long enough for microscopic life to emerge.[15]

Soil and rock samples studied in 2013 by NASA's microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively.[17]

Impact Glass, shown to preserve signs of life on Earth, was discovered on Mars, and could contain signs of ancient life, if life ever existed.[26]

Present

No definitive evidence for biosignatures or organics of Martian origin has been identified, and assessment will continue not only through the Martian seasons, but also back in time as the Curiosity rover studies what is recorded in the depositional history of the rocks in Gale Crater.[11] While scientists have not identified the minimum number of parameters for determination of habitability potential, some teams have proposed hypotheses based on simulations.

Subsurface

Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms,[11] life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen.[27][28][29][30][31][32] The radiation environment on the surface, as recently determined by Curiosity rover "is so high that any biological organisms would not survive without protection."[33] Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet.[17][32][34][35][36][37][38] The extensive geothermal heat – potentially providing a habitable environment away from the harsh surface conditions.[32][39][40][41]

Surface brines

Although liquid water does not appear at the surface of Mars,[42] there is conclusive evidence of hydrated halophile psychrophilic).[46] Several biologists argue that although chemically important, thin films of transient liquid brine are not likely to provide suitable sites for life, as the activity of water on salty films, the temperature, or both are less than the biological thresholds across the entire Martian surface and shallow subsurface.[13][47][48][49][50]

^ abcThan, Ker (January 29, 2007). "Study: Surface of Mars Devoid of Life". Space.com. After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation.

^ abcJohnThomas Didymus (January 21, 2013). "Scientists find evidence Mars subsurface could hold life". Digital Journal – Science. There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell.

^Gronstal, Aaron (May 15, 2014). "Destroying Glycine in Ice". NASA Astrobiology. Retrieved 2014-08-13. To date, we have not left the top-most surface of Mars, and the radiation environment there (as recently determined by Curiosity) is so high that any biological organisms would not survive without protection.

^ abSteigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center (NASA). Archived from the original on 2009-01-16. If microscopic Martian life is producing the methane, it probably resides far below the surface, where it's still warm enough for liquid water to exist

^"Mars Rovers Sharpen Questions About Livable Conditions". NASA. Archived from the original on 2008-02-18.

^Anderson, Paul S. (December 15, 2011). "New Study Says Large Regions of Mars Could Sustain Life". Universe Today. Most scientists would agree that the best place that any organisms could hope to survive and flourish would be underground.

^"Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 2013-06-06. If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.

^Klotz, Irene (13 April 2015). "Briny Water May Pool in Mars' Equatorial Soil". Discovery News. Retrieved 2015-04-14. “It is not just a problem of water, but also temperature. The water activity and temperatures are so low in Mars that they are beyond the limits of cell reproduction and metabolism”

^Pearson, Michael (14 April 2015). "Briny puddles could dot Mars, new research says". CNN. Retrieved 2015-04-15. The new study doesn't change the picture for life on Mars. The researchers say the temperatures they measured are too low and water too scarce "to support terrestrial organisms"

^Lovet, Richard A. (February 2, 2007). "Mars Life May Be Too Deep to Find, Experts Conclude". National Geographic News. That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere.

^ abStern, Jennifer C. (24 March 2015). "Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale crater, Mars". Proceedings of the National Academy of Sciences of the United States of America (PNAS)112: 4245–4250.

^Committee on an Astrobiology Strategy for the Exploration of Mars; National Research Council (2007). "Planetary Protection for Mars Missions". An Astrobiology Strategy for the Exploration of Mars. The National Academies Press. pp. 95–98.

^Biemann, Klaus (2007). "On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter". Proceedings of the National Academy of Sciences of the United States of America104 (25): 10310–10313.

^Navarro-González, R.; McKay, C. P. (2011). "Reply to comment by Biemann and Bada on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars"". Journal of Geophysical Research116.

Mars Sample Return Mission — The best life detection experiment proposed is the examination on Earth of a soil sample from Mars. However, the difficulty of providing and maintaining life support over the months of transit from Mars to Earth remains to be solved. Providing for still unknown environmental and nutritional requirements is daunting. Should dead organisms be found in a sample, it would be difficult to conclude that those organisms were alive when obtained.

On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of chlorobenzene, were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover.[130][131]

Mars Science Laboratory

[233] in contact with liquid water would have formed only CaSO4. This suggests a severely arid environment, with minimal or no liquid water interaction.2)4 in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO2)4 In addition, recent analysis of the Phoenix WCL showed that the Ca(ClO[232]
The

An artist's concept of the Phoenix spacecraft

Phoenix lander, 2008

After the discovery of [224] Biemann has written a commentary critical of this Navarro-González paper as well,[225] to which the latter have replied;[226] the exchange was published in December 2011.

Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, so Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection.
[222] wrote a rebuttal.Viking, the principal investigator of the GCMS experiment on Klaus Biemann [160]
A research team from the

In 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, D.C., USA), Gilbert Levin's investigation was assessed once more.[153] Levin still maintains that his original data were correct, as the positive and negative control experiments were in order.[219] Moreover, Levin's team, on 12 April 2012, reported a statistical speculation, based on old data —reinterpreted mathematically through cluster analysis— of the Labeled Release experiments, that may suggest evidence of "extant microbial life on Mars."[219][220] Critics counter that the method has not yet been proven effective for differentiating between biological and non-biological processes on Earth so it is premature to draw any conclusions.[221]

One of the designers of the Labeled Release experiment, detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover.[130][131] Nonetheless, the results of the Viking mission concerning life are considered by the general expert community, at best, as inconclusive.[10][217][218]

A 2011 astrobiology textbook notes that the GCMS was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that the Viking missions failed to detect life in the Martian soil."[214]

detected no organic molecules. However, there are vastly different interpretations of what those results imply.
GCMS was evolved in the Labeled Release experiment, and that the 2CO14 production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled 2CO14 The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result, showing increased [213]
The primary mission of the

Viking experiments

Liquid water is necessary for known life and metabolism, so if water was present on Mars, the chances of it having supported life may have been determinant. The Viking orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams.[210][211][212]

Viking orbiters

Mariner 4 probe performed the first successful flyby of the planet Mars, returning the first pictures of the Martian surface in 1965. The photographs showed an arid Mars without rivers, oceans, or any signs of life. Further, it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4 billion years. The probe also found that Mars has no global magnetic field that would protect the planet from potentially life-threatening cosmic rays. The probe was able to calculate the atmospheric pressure on the planet to be about 0.6 kPa (compared to Earth's 101.3 kPa), meaning that liquid water could not exist on the planet's surface.[10] After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.

Streamlined Islands seen by Viking orbiter showed that large floods occurred on Mars. Image is located in Lunae Palus quadrangle.

Mariner 4

Mars-1 was the first spacecraft launched to Mars in 1962,[208] but communication was lost while on route to Mars. With Mars-2 and Mars-3 in 1971-1972, information was obtained on the nature of the surface rocks and altitude profiles of the surface density of the soil, its thermal conductivity, and thermal anomalies detected on the surface of Mars. The Program found that its northern polar cap has a temperature below -110 °C and that the water vapor content in the atmosphere of Mars is five thousand times less than on Earth. No signs of life were found.[209]

Mars-2

Missions

On 26 April 2012, scientists reported that an extremophilelichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[202][203][204][205][206][207] However, the ability to survive in an environment is not the same as the ability to thrive, reproduce, and evolve in that same environment, necessitating further study.

Life under simulated Martian conditions

The prime concern of hardware contaminating Mars, derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (Deinococcus radiodurans and genera Brevundimonas, Rhodococcus, and Pseudomonas under simulated Martian conditions.[199] Results from one of these experimental irradiation experiments, combined with previous radiation modeling, indicate that Brevundimonas sp. MV.7 emplaced only 30 cm deep in Martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10⁶ population reduction.[199] Surprisingly, the diurnal Mars-like cycles in temperature and relative humidity affected the viability of Deinococcus radiodurans cells quite severely.[200] In other simulations, Deinococcus radiodurans also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.[201]

[197]

Forward contamination

A multinational European team suggests that if liquid water is present in the spiders' channels during their annual defrost cycle, they might provide a niche where certain microscopic life forms could have retreated and adapted while sheltered from solar radiation.[193] A British team also considers the possibility that [194] It has been proposed to develop the Mars Geyser Hopper lander to study the geysers up close.[195]

[192] perspective.astrobiological promoted these formations as deserving of study from an Arthur C. Clarke Since their discovery, fiction writer [191][190] The Hungarian scientists believe that even a complex sublimation process is insufficient to explain the formation and evolution of the dark dune spots in space and time.[189][188][187]
A team of Hungarian scientists proposes that the geysers' most visible features, dark dune spots and spider channels, may be colonies of

The seasonal frosting and defrosting of the southern ice cap results in the formation of spider-like radial channels carved on 1 meter thick ice by sunlight. Then, sublimed CO2 – and probably water – increase pressure in their interior producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.[183][184][185][186] This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars.

Geysers on Mars

Yamato 000593 is the second largestmeteorite from Mars found on Earth. Studies suggest the Martian meteorite was formed about 1.3 billion years ago from a lava flow on Mars. An impact occurred on Mars about 12 million years ago and ejected the meteorite from the Martian surface into space. The meteorite landed on Earth in Antarctica about 50,000 years ago. The mass of the meteorite is 13.7 kg (30 lb) and has been found to contain evidence of past water movement.[180][181][182] At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by biotic activity according to NASA scientists.[180][181][182]

Yamato 000593

The Shergotty meteorite, a 4 kg Martian meteorite, fell on Earth on Shergotty, India on August 25, 1865 and was retrieved by witnesses almost immediately.[179] This meteorite is relatively young, calculated to have been formed on Mars only 165 million years ago from volcanic origin. It is composed mostly of pyroxene and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest remnants of a biofilm and its associated microbial communities.[167] Work is in progress on searching for magnetites within alteration phases.

Shergotty

This caused additional interest in this meteorite, so in 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic carbon content was observed. When the results and evidence were published on 2006, some independent researchers claimed that the carbon deposits are of biologic origin. However, it was remarked that since carbon is the fourth most abundant element in the Universe, finding it in curious patterns is not indicative or suggestive of biological origin.[177][178]

In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects[175] of the size and shape consistent with Earthly [167][176]

Nakhla

Some structures resembling the mineralized casts of terrestrial bacteria and their appendages (fibrils) or by-products (extracellular polymeric substances) occur in the rims of carbonate globules and preterrestrial aqueous alteration regions.[171][172] The size and shape of the objects is consistent with Earthly fossilizednanobacteria, but the existence of nanobacteria itself is controversial.

The meteorite displays indication of relatively low temperature secondary mineralization by water and shows evidence of preterrestrial aqueous alteration. Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface.

Nakhla meteorite

The biogenic explanation is a more viable hypothesis for the origin of the magnetites in the meteorite.[169][170]

ALH84001

As of 2010, reexaminations of the biomorphs found in the three Martian meteorites are underway with more advanced analytical instruments than previously available.

For general acceptance of past life in a geologic sample, essentially most or all of these criteria must be met. All seven criteria have not yet been met for any of the Martian samples, but continued investigations are in progress.[167]

Is the geologic context of the sample compatible with past life?

Is the age of the sample and its stratigraphic location compatible with possible life?

Does the sample contain evidence of cellular morphology and colonies?

Is there any evidence of biominerals showing chemical or mineral disequilibria?

Is there any evidence of stable isotope patterns unique to biology?

Are there any organic biomarkers present?

Are the features indigenous to the sample?

Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:[167]

biomorphs). Although the scientific evidence collected is reliable, its interpretation varies. To date, none of the original lines of scientific evidence for the hypothesis that the biomorphs are of exobiological origin (the so-called biogenic hypothesis) have been either discredited or positively ascribed to non-biological explanations.[167]

Meteorites

The Labeled Release evidence was not generally accepted initially, and, to this day lacks the consensus of the scientific community.[165]

[164][163] indicate that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2. The study determined that [160][159][158][157]
Relatively recent assessments published in December 2010 by Rafael Navarro–Gonzáles,

that the Viking LR experiments did, indeed, detect extant microbial life on Mars.
[156]
The 1970s

Viking lander biological experiments

In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter had detected traces of formaldehyde in the atmosphere of Mars. Vittorio Formisano, the director of the PFS, has speculated that the formaldehyde could be the byproduct of the oxidation of methane and, according to him, would provide evidence that Mars is either extremely geologically active or harbouring colonies of microbial life.[147][148] NASA scientists consider the preliminary findings well worth a follow-up, but have also rejected the claims of life.[149][150]

Formaldehyde

India's Mars Orbiter Mission, launched on November 5, 2013, is searching for methane in the atmosphere of Mars using its Methane Sensor for Mars (MSM). The orbiter has been orbiting Mars since September 24, 2014. The ExoMars Trace Gas Orbiter planned to launch in 2016 would further study the methane,[145][146] as well as its decomposition products such as formaldehyde and methanol.

The detected a "tenfold spike" in the level of methane in the Martian atmosphere compared to the usual background readings.[130][131][132] However, even if the mission is to determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside the rover's reach.[133] The first measurements with the Tunable Laser Spectrometer (TLS) in the Curiosity rover indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.[134][135][136][137] On July 19, 2013, NASA scientists published the results of a new analysis of the atmosphere of Mars, reporting a lack of methane around the landing site of the Curiosity rover.[138][139][140] On September 19, 2013, NASA again reported no detection of atmospheric methane with a measured value of 6999180000000000000♠0.18±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, concluded that the probability of current methanogenic microbial activity on Mars is reduced.[141][142][143] On 16 December 2014, NASA reported that Curiosity had detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7 ppb, suggesting that methane is released at intervals.[130][131][144]

Research at the University of Arkansas presented in June 2015 suggested that some methanogens could survive on Mars's low pressure. Rebecca Mickol, found that in her laboratory, four species of methanogens survived low-pressure conditions that were similar to a subsurface liquid aquifer on Mars. The four species that she tested were Methanothermobacter wolfeii, Methanosarcina barkeri, Methanobacterium formicicum, and Methanococcus maripaludis.[122] In June 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.[126][127] According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active."[126] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.[128][129]

A team led by Levin suggested that both phenomena—methane production and degradation—could be accounted for by an ecology of methane-producing and methane-consuming microorganisms.[124][125]

Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and in vitro experiments testing growth of methanogenic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0wt% perchlorate salt.[123] The results reported indicate that the perchlorates discovered by the Phoenix Lander would not rule out the possible presence of methanogens on Mars.[123][124]

[34] If microscopic Martian life is producing the methane, it probably resides far below the surface, where it is still warm enough for liquid water to exist.[122]
The existence of life in the form of

Distribution of methane in the atmosphere of Mars in the Northern Hemisphere during summer

Trace amounts of ultraviolet radiation.[121]

Methane

Possible biosignatures

Based on Earth analogs, biosignatures.[105][106][107] For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.[108][109][110]

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in silica (90%).[103] The feature is reminiscent of the effect of hot spring water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life, and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.[104]

Silica

There is disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.[95][96][97][98] Others suggest it may be liquid brine near the surface,[99][100][101] but the exact source of the water and the mechanism behind its motion are not understood.[102]

In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.[93][94] Additional similar images were published in 2006, taken by the Mars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in steep crater walls and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago.

Analysis of Martian sandstones, using data obtained from orbital spectrometry, suggests that the waters that previously existed on the surface of Mars would have had too high a salinity to support most Earth-like life. Tosca et al. found that the Martian water in the locations they studied all had water activity, aw ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.[91]Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.[92]

Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.[76][77] A small amount of water vapor is present in the atmosphere.[78] There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago,[79] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,[80][81][82][83] including large oceans.[84][85][86][87][88] It has been estimated that the primordial oceans on Mars would have covered between 36%[89] and 75% of the planet.[90]

Liquid water, necessary for life as we know it, cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.[67][68] Liquid water does not appear at the surface itself,[69] but it could form in minuscule amounts around dust particles in snow heated by the Sun.[70][71] Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.[72][73][74][75]

A series of artist's conceptions of past water coverage on Mars.

Liquid water

Further complicating estimates of the habitability of the Martian surface is the fact that very little is known on the growth of microorganisms at pressures close to the conditions found on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar).[66] In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only Serratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0 °C, and CO2-enriched anoxic atmospheres.[66]

Low pressure

[65]
On 24 March 2015, NASA reported that the

After carbon, nitrogen is arguably the most important element needed for life. Thus, measurements of nitrate over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N2) in the atmosphere at low levels, but this is not adequate to support nitrogen fixation for biological incorporation.[61] Nitrogen in the form of nitrate could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed by thermal shock from impact or volcanic plume lightning on ancient Mars.[62]

Nitrogen fixation

Data collected by the [60]

Even the most radiation-tolerant Earthly bacteria would survive in dormant spore state only 18,000 years at the surface; at 2 meters —the greatest depth at which the ExoMars rover will be capable of reaching— survival time would be 90,000 to half million years, depending on the type of rock.[29]

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.[54] Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years.[55] As a result, the planet has been vulnerable to radiation from space for about 4 billion years.[56] Currently, ionizing radiation on Mars is typically two orders of magnitude (or 100 times) higher than on Earth.[57] Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars for that long.[27][58] After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.[27][28][59] The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 metres below the planet's surface.[28]

Cosmic radiation

[53]

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